In U.S. Pat. No. 3,453,356, there is disclosed a twin-screw extrusion device in which additives can be admixed with a fused thermoplastic plastics material. In this prior art arrangement, a thermoplastic plastics material is introduced into an extruder barrel first through a feed aperture and is then fused. Pulverized glass fibers are then introduced into the fused plastics material through a second feed aperture disposed downstream of the first feed aperture.
Helical flights are provided around the extrusion screw disposed in the second feed aperture. Such flights occupy a large proportion of the internal volume of the barrel. The conveying of material in an extrusion device is brought about by the adhesion of the melt to the interior wall of the barrel and, substantially simultaneously the melt is scraped from such wall by the helical flight on the screw. It is generally true to say that the better the material adheres to the interior wall of the barrel, the better the conveying capacity of the helical screw flights conveying the material.
Since a considerable portion of the internal surface of the barrel which assists in conveying the material is absent in the area occupied by the second feed aperture, the conveyance of material is often very unsatisfactory in this particular region of the extrusion device. A large proportion of the molten material remains in the screw threads and acts as a block or undergoes laminar flow without performing a rolling movement on the internal wall of the barrel.
There is an increasing demand for plastics materials having differing properties. Special additives, such as carbon black in the form of powder or fibers and the like, are required to be mixed with the plastics material in order to impart the desired properties to the latter. Considerable problems are involved in, for example, mixing free carbon black in powder form with molten polymers and also in obtaining a mixture which is highly homogeneous. Absolute homogeneity is of prime importance particularly when a thin foil, having a thickness of, for example 25 m.mu., has to be produced from the mixed plastics material.
On the other hand, even from an extruder used for producing specialist plastics materials, a high output is of great financial importance because the capital outlay necessitated by such an extrusion system is very high. Hitherto, very long extruders have been used to obtain satisfactory homogeneity of the mixture of plastics material and additives. In theory, increasing the length of the extrusion device prolongs the mixing time and this improves the homogeneity. However, increasing the length of the extrusion device increases the capital outlay more than would be expected.